The invention comprises a method for preparing a high temperature superconducting cuprate material (HTSC) to maximise the critical current density of the material, in which the doping state or hole concentration of the material is controlled so as to lie at about the point where the normal-state pseudogap reduces to a minimum.
Many High-Tc Superconducting Cuprates (HTSC) are known to have superconducting transition temperatures, Tc exceeding the temperature at which liquid nitrogen boils, 77 K. As such they have a potentially large number of applications ranging from power generation, distribution, transformation and control, to high-field magnets, motors, body scanners, telecommunication and electronics. Tc values may be of the order of 93 K for example for YBa2Cu3O7xe2x88x92xcex4, 95K for example for Bi2Sr2CaCu2O8, 109 K for example for Bi2Sr2Ca2Cu3O10, 120K for example for TlBa2Ca2Cu3O10 or as high as 134 K for HgBa2Ca2Cu3O10. For many of these applications such Tc values alone do not guarantee the utility of these HTSC at 77K or higher temperatures. Often these applications require large critical current densities, Jc, in the presence of a magnetic field. Even if the grains of the HTSC are crystallographically aligned, otherwise known as textured, and well sintered together, as is commonly achieved in thin-films, such that weak links between the grains are removed, a high critical current in the presence of a magnetic field is not guaranteed. Such high currents are only achieved if there is strong flux pinning within the individual grains. A simple measure of the flux pinning contribution to Jc is the product, Uo"xgr", of the condensation energy, Uo, and the superconducting coherence length, "xgr". An additional measure of the intrinsic ability of HTSC materials to support high critical currents in the presence of a magnetic field is the temperature-dependent irreversibility field, H*(T). For magnetic fields exceeding H* the magnetisation is reversible and hence dissipative while it is irreversible for fields less than H* and hence substantially non-dissipative. If for a given temperature H* is large then at that temperature the critical current density for fields H less than H* may be high. Many models for the irreversibility field have H*xcex1xcexLxe2x88x922 where xcexL is the in-plane London penetration depth. Thus if xcexL is minimised (xcexxe2x88x922 maximised) then H* may be maximised.
HTSC have a fundamentally important feature that their properties vary with the concentration of doped electronic carriers. The carriers are electron holes, referred to in short as holes. The concentration of holes may be altered by chemical substitution or by changing the oxygen concentration. In general, the hole concentration may be increased by substituting a lower valency atom for a higher valency atom or by increasing the oxygen content. The hole concentration, p, may be decreased by substituting a higher valency atom for a lower valency atom or by decreasing the oxygen content. Thus in La2CuO4 which is an undoped insulator, the hole concentration is increased from zero by substituting Sr2+ for the La3xe2x88x92. In YBa2Cu3O7 the hole concentration may be decreased by substituting La3+ for Ba2+ or by decreasing the oxygen content as in the formula YB2Cu3O7xe2x88x92xcex4 where xcex4 may be increased from 0 to 1. When xcex4=1 this compound in an undoped insulator like La2CuO4. By increasing the hole concentration from the undoped insulating state the HTSC eventually becomes superconducting at low temperature. This (lower) threshold hole concentration is about p=0.05, for example. If the hole concentration is increased too high beyond an upper threshold then the HTSC material becomes a non-superconducting metal even at the lowest temperature. This upper threshold is about p=0.27. Between the lower and upper thresholds Tc rises up smoothly from zero at the lower threshold to a maximum at about p=0.16 then falls smoothly back to zero at the upper threshold. The maximum Tc value in this variation with hole concentration is Tc,max and it occurs at a hole concentration frequently referred to as optimal doping. The variation in Tc with hole concentration follows a nearly parabolic variation approximated by Tc(p)=Tc,max[1xe2x88x9282.6(pxe2x88x920.162], so that as noted Tc maximises at p≈0.16. At optimal doping the room temperature thermoelectric power takes the value Q(290K)=+2 xcexcV/K (Tallon et al, U.S. Pat. No. 5,619,141 which is incorporated herein by reference). When the hole concentration is less than optimal doping the HTSC material is referred to as underdoped and when it is greater than optimal doping it is referred to as overdoped. Optimal doping then is seen as the key doping state to which other doping states are referred. Prior to the present invention the usual criterion for optimising superconductivity in HTSC was to maximise Tc.
In broad terms in one aspect the invention comprises a method for preparing a high temperature superconducting cuprate material (HTSC) to maximise the critical current density (Jc) thereof, comprising the step of controlling the doping state or hole concentration of the material to be higher than the doping state or hole concentration of the material that provides a maximum superconducting transition temperature (Tc) to increase the critical current density of the material.
In broad terms in another aspect the invention comprises a high temperature superconducting cuprate material (HTSC) having a doping state or hole concentration higher than the doping state or hole concentration of the material for maximum superconducting transition temperature (Tc) and at about a value where the normal-state pseudogap for the material reduces to a minimum and which maximises the critical current density (Jc) of the material.
We have surprisingly found that the optimal doping for maximising Tc is not the optimal doping for maximising flux pinning, Uo, xcexLxe2x88x922 or the critical current density Jc. We have found that JC, flux pinning, Uo, xcexLxe2x88x922 maximise at a higher doping state or hole concentration than that which maximises Tc and at about the point where the normal-state pseudo-gap reduces to a minimum, particularly where the hole concentration 0.18xe2x89xa6pxe2x89xa60.20 and most particularly at about p=0.19 or p=0.19xc2x10.005. We believe this is due to the presence, across the underdoped and slightly overdoped regions, of a correlated state that reduces low-energy spectral weight in the quasiparticle excitation spectrum. This reduction in spectral weight is referred to as the pseudogap and is sometimes mistakenly described as a spin gap. The pseudogap is manifested as a reduction in the normal-state entropy and susceptibility which strongly suppresses superconductivity and all measures thereof including Tc, condensation energy and superfluid density. The pseudogap energy, Eg, may be determined by fitting the normal state temperature dependence of the Knight shift, Ks(T), to an equation Ks(T)=Ko(1xe2x88x92tan h2 (Eg/2kT)+Kc where Kc is the chemical shift.
Eg may alternatively be determined by fitting the normal-state temperature dependence of the entropy, S(T) to an equation S(T)/T=go[tan h2(Eg/2kT)], where go is a constant.
Typically Eg is found to decrease with increasing hole concentration, p. More sophisticated means of determining Eg are known (see Tallon et al J. Phys. Chem. Solids 59, 2145 (1998) which is incorporated herein by reference) but the values of Eg thus determined still reduce approximately linearly with increasing hole concentration and, most importantly, still reduce to zero at about the same value p=0.19xc2x10.005.
The method may include overdoping the HTSC so that the grain boundary regions between individual grains in the HTSC in particular are doped to maximise the critical current density across grain boundaries. Such grain boundary regions can tend to be underdoped even if the bulk intragranular material is optimally doped or even overdoped (xe2x80x94see Babcock et al (Physica C 227, 183 (1994)). The pseudogap will often be locally present in the grain boundary regions, the effective superconducting order parameter thus locally reduced, and the grains weakly linked. Additionally, impurities tend to accumulate at the grain boundaries. HTSC have a d-wave order parameter which is very sensitive to the presence of impurities Tc being rapidly suppressed at a rate dTc/dy that depends strongly on doping state. In the underdoped grain boundary regions Tc, the order parameter and the condensation energy are all much more rapidly suppressed due to impurity scattering than in the bulk intragranular material. Impurities in underdoped grain boundaries are therefore especially deleterious. Thus while the intragranular Jc may be high, the intergranular Jc may be low due to the underdoped state of the grain boundary regions.
The method of the invention may be used in producing HTSC as bulk materials, wires, tapes or other conductor elements, or thick or thin films for example.